1. Field of the Invention
The present invention relates generally to a method for measuring time dependent signals by focusing a gated particle probe on a measuring point, as well as to an apparatus for practicing the method.
2. Description of the Prior Art
Checking the operation of VLSI (very large scale integrated) circuits is usually carried out in a computer-controlled test system so that malfunctions of a circuit module or chip are identified by measuring and analyzing the output signals therefrom, which result from bit patterns supplied to the module. In most cases, however, such test are incomplete since faulty circuit parts cannot be localized or, if they can be localized, it is only with a great expense and a highly complex apparatus. Additional measurements must therefore be made inside the circuit modules for fault identification, particularly during the development phase of the modules or chips. Electron beam measuring methods have proved particularly suitable for such purposes and are utilized in all regions of development and manufacture of microelectronic components. A person skilled in the art is familiar with the most widespread methods and practice, such as from the publication of H. P. Feuerbaum, "Electron Beam Testing: Methods and Applications", Scanning, Volume 5, pages 14-24 (1983).
Measuring the quantitative potential along a chronological curve at selected nodes of a component under test provides especially illuminating indications for localizing faults in VLSI circuits. This method is known, for example, from the publication by H. P. Feuerbaum, "VLSI Testing Using the Electron Probe", Scanning Electron Microscopy, 1971/1, pages 285-296. Disclosed therein is a primary electron beam generated in an electron-optical column of a modified scanning electron microscope which is positioned at a measuring point. The shift in energy distribution of secondary electrons triggered at the measuring point is identified using a spectrometer-detector arrangement, where the energy shift is dependent upon the potential of the component at the measuring point.
Quantitative measurements of a potential curve having a chronological resolution in the nanosecond range are only stroboscopically possible in accordance with the sampling principle. The sampling method, as used in electrical measuring technology, provides a primary electron beam pulsed synchronously at the frequency of a signal to be measured. The curve of the potential is continuously registered by shifting the keying time of the primary electron pulses. A high chronological resolution is achieved through the use of short primary electron pulses.
For shorter pulse durations, however, the signal level at the output of the spectrometer detector arrangement decreases greatly so that there is a high demand made on the measuring electronics. Since pulse widths of 100 picoseconds and probe currents of one nanoampere are obtainable with conventional beam blanking systems, the primary electron pulses statistically contain less than one electron. Thus, the current measurement usually carried out requires extremely high amplification.
Attempts have been made to further enhance the documentation sensitivity which defines the shortest pulse duration that results in an evaluatable signal and, thus, a possible chronological resolution. Such attempts have included enhancing the signal with the assistance of sensitive detectors and electron sources having highly directional characteristics (brightness). However, even when sample and hold methods are used, the highly amplified noise of the electronic components ultimately limits the obtainable documentation limit. This in turn limits the maximum possible chronlogical resolution.